The present invention relates to dichroic dyes and, more particularly, to novel dichroic dyes of the (substituted naphthylazo) azobenzene type, each having three azo bonding groups and useful in a liquid crystal mixture for providing contrast ratios on the order of 10:1.
A display, particularly one of the liquid crystal type, can be characterized by brightness and contrast performance criteria. It is known to improve these performance criteria by dissolving a guest dichroic dye in a host liquid crystal material. Many dyes have been especially developed for a variety of applications such as dyeing of fabrics, printing of textiles, coloring of plastics, color image formation in photography, etc. To provide the requisite properties, such as hue, solubility, affinity for the substrate, chemical resistance, and compatibility with the medium from which the dye is applied, the molecular structure and bond rigidity of the dye are specially designed for each application. Important properties required for the present application in liquid crystal displays, include the following: dichroism, solubility and high order parameter.
Dichroism is the property whereby an oriented assembly of dye molecules exhibits relatively low absorption of a given wavelength of light in one state of orientation and a relatively high absorption of the same wavelength in another state of orientation with respect to the light source. The orientation can be brought about by dissolution of the dye in a liquid crystal solvent or by embedding the dye in a stretched plastic.
Solubility must be sufficiently high so that thin layers, for example, of ten micrometers, have adequate light absorption in one of the oriented states. Ionic dyes will generally be inadequate not only because of their low solubility but also because they increase the electrical conductivity of liquid crystals.
Order parameter is a quantitative measure of the degree of molecular order or alignment in a given system. High order parameter is promoted by dyes with elongated shape having a large ratio of molecular length to breadth, similar to the shape of the molecules of liquid crystal host material. To assure an elongated shape, the molecules should have a rigid structure which can be obtained, for example, by linking benzene or heterocyclic rings with double-bonded groups. The brightness and contrast of the display are both related to the order parameter S of the dye, where S=(r-1)/(R+2) and R is the ratio of light absorption in the dye measured with a polarizer respectively parallel and perpendicular to the nematic director of the liquid crystal host at the wavelength of maximum absorption. Advantageously, the order parameter should be at least 0.70, and preferably as high as possible, to achieve a minimum desired contrast ratio on the order of 10:1, while still allowing a liquid crystal guest-host display to be fabricated with a reasonable brightness parameter.